EXCHANGE 


A  Study  of  the  Relation  of  the  Structure 
of    J-Mercaptobenzothiazole   and  its  De- 
rivatives to  Their  Value  as  Accelerators 
of  Rubber  Vulcanization 


DISSERTATION 

PRESENTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIRE- 
MENTS FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 
IN  THE  GRADUATE  SCHOOL  OF  THE  OHIO 
STATE  UNIVERSITY 


BY 

LORIN  BERYL  SEBRELL 


THE  OHIO  STATE  UNIVERSITY 
1922 


A  Study  of  the  Relation  of  the  Structure 
of    J-Mercaptobenzothiazole   and   its  De- 
rivatives to  Their  Value  as  Accelerators 
of  Rubber  Vulcanization 


DISSERTATION 

PRESENTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIRE- 
MENTS FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 

IN  THE  GRADUATE  SCHOOL  OF  THE  OHIO 
STATE  UNIVERSITY 


BY 

LORIN  BERYL  SEBRELL 


THE  OHIO  STATE  UNIVERSITY 
1922 


TABLE  OF  CONTENTS 

Page 

Introduction 4 

Part  I — Preparation  and  Properties 

Historical 6 

Mechanism  of  the  reaction 6 

Preparation  of  materials 8 

Apparatus .-  •  •  •  9 

Procedure 9 

Discussion  of  experimental  results. . . 9 

Derivatives  of  1-mercaptobenzothiazole  and  their  physical  properties 12 

Summary  and  conclusions 

Part    II — 1-Mercaptobenzothiazole    and    its    Derivatives  as  Accelerators  of  Rubber 
Vulcanization  • 

Historical «  15 

Derivatives 15 

Results  of  compounding  tests 17 

Mechanism  of  acceleration  by  benzothiazole  derivatives 25 

Summary  and  conclusions : 28 

Acknowledgment 28 

Autobiography 29 


545101 


INTRODUCTION 

By  an  accelerator  of  rubber  vulcanization  is  meant  any  compound 
which  when  added  to  the  rubber-sulfur  mixture  not  only  shortens  the 
time  required  for  vulcanization  but  also  causes  a  decided  change  in  the 
resulting  physical  properties. 

The  first  organic  compound  to  be  so  used  was  aniline,  in  the  year 
1906.  The  use  of  this  compound  by  Oenslager  and  Marks  in  Akron 
was  the  beginning  of  one  of  the  most  important  periods  in  the  history  of 
the  rubber  industry.  Since  that  time  many  hundreds  of  different  or- 
ganic compounds  have  been  tried  with  varying  degrees  of  success.  The 
exact  mechanism  by  which  these  accelerators  bring  about  the  desired 
results  has  not  been  definitely  determined  in  all  cases,  although  con- 
siderable progress  has  been  made. 

The  present  investigation  consists  of  two  parts.  The  first  deals  with 
the  methods  of  preparation  and  the  properties  of  a  recently  discovered 
series  of  accelerators.  The  second  part  is  concerned  with  the  use  of 
these  compounds  in  rubber,  the  relation  of  their  structure  to  their  curing 
Dower,  and  the  mechanism  by  which  they  function  as  accelerators. 


PART  I 

THE    PREPARATION    AND    PROPERTIES     OF     1-MERCAPTO- 
BENZOTHIAZOLE,  ITS  HOMOLOGS  AND  DERIVATIVES1 2 

The  present  paper  comprises  a  study  of  the  synthesis  of  1-mercapto- 
benzothiazole  and  its  derivatives  which  was  made  in  connection  with 
an  investigation  of  the  role  played  by  these  compounds  when  functioning 
as  accelerators  of  vulcanization.  Eight  different  mercapto-benzothiazoles 
were  prepared  and  studied.  Each  of  these  substances  excepting  where 
the  quantity  of  material  was  limited,  were  prepared  by  four  separate 
methods.  These  methods  were  alike  in  that  the  reaction  mixture  was 
heated  in  an  autoclave  under  pressure.  The  reaction  mixtures  were  as 
follows:  (1)  the  corresponding  disubstituted  thio-urea  and  sulfur;  (2) 
the  zinc  salt  of  the  corresponding  aryl  dithiocarbamic  acid  and  sulfur; 
(3)  the  ammonium  salt  of  the  same  acid  and  sulfur;  (4)  a  mixture  of  the 
corresponding  aryl  amine,  carbon  disulfide  and  sulfur. 

The  first  method  has  been  described  by  Romani.3  The  last  three 
methods  are  new. 

1  /.  Amer.  Chem.  Soc.  45,  2390. 

2  The  system  of  nomenclature  as  outlined  in  C.  A.  Decennial  Index,  1-10,  2345, 
namely,  omitting  the  sulfur  and  beginning  the  numbering  with  the  carbon  atom  of  the 
thiazole  ring  has  been  followed  throughout  this  article. 

6 


2 

3  The  four  methods  of  preparation  described  in  the  following  pages  as  applied  to 
1-mercapto-benzothiazole  and  its  three  monomethyl  derivatives,  together  with  their 
disulfides  and  metallic  salts  were  reported  before  the  Organic  Division  of  the  American 
Chemical  Society  at  the  Birmingham  meeting,  April  6th,  1922.  [(a)  See  Science,  56, 
55  (1922).]  Shortly  afterward,  an  article  by  Romani,  [(b)  Gazz.  chim.  ital,  52,  29 
(1922)]  became  available,  describing  the  preparation  of  the  three  methyl  derivatives  by 
one  of  the  methods,  namely,  heating  the  corresponding  disubstituted  thio-urea  with 
sulfur.  To  him,  undoubtedly,  belongs  priority  of  publication  of  a  description  of  these 
three  derivatives  by  this  one  method.  He  did  not  describe  the  disulfides. 


Historical 

l-Mercapto-benzothiazole  was  first  obtained  by  A.  W.  Hofmann4  in  an  attempt  to 
prepare  the  disulfhydryl  derivative  of  thiocarbanilide  by  the  action  of  carbon  disulfide 
on  o-aminophenol.  He  obtained  the  same  substance  by  the  action  of  sodium  hydro- 
sulfide  on  chlorophenyl  mustard  oil  (1-chloro-benzothiazole)  and  also  when  carbon  di- 
sulfide was  caused  to  react  with  0-aminothiophenol  disulfide.  The  product  thus  ob- 
tained, after  recrystallization  from  alcohol,  melted  at  179°  and  was  easily  oxidized  to  a 
disulfide  melting  at  180°. 

Jacobson  and  Frankenbacher5  while  studying  the  formation  of  benzothiazoles, 
heated  azobenzene  with  carbon  disulfide  in  a  sealed  tube  at  250°  for  5  hours.  The  prod- 
uct melted  at  174°  but  was  identical  with  Hofmann's  1-mercapto-benzothiazole.  The 
disulfide  obtained  by  the  oxidation  of  this  product  with  potassium  dichromate  in  acetic 
acid  solution  after  recrystallization  from  benzene  melted  at  186°. 

In  order  to  verify  the  assumption  that  phenyl  mustard  oil  is  an  intermediate  prod- 
uct in  the  formation  of  1-mercapto-benzothiazole  from  azobenzene,  these  authors  heated 
the  former  substance  in  a  sealed  tube  with  sulfur  for  5  hours.  The  yield  of  mercapto- 
benzothiazole  thus  obtained  was  equal  to  45%  of  the  weight  of  mustard  oil  used.  The 
constitution  of  the  thiazole  was  further  established  by  fusion  with  potassium  hydroxide, 
thus  regenerating  o-aminothiophenol. 

Azobenzene  when  heated  with  phenyl  mustard  oil  was  found  to  yield  1-anilido- 
benzothiazole  although  the  same  substance  could  not  be  obtained  by  the  direct  action  of 
aniline  on  mercapto-benzothiazole. 

Rassow,  Dohle  and  Reim6  have  shown  that  1-mercapto-benzothiazole  is  formed  by 
the  action  of  sulfur  on  dimethylaniline.  , 

Bedford  and  Sebrell7  as  well  as  Bruni  and  Romani8  have  independently  described 
the  preparation  of  1-mercapto-benzothiazole  by  the  reaction  of  thiocarbanilide  with  sulfur 
when  heated  under  pressure.  More  recently  Romani3b  has  extended  this  method  of 
preparation  to  the  three  monomethyl  derivatives  of  1-mercapto-benzothiazole.  He 
recommends  the  use  of  an  excess  of  sulfur  and  zinc  oxide  as  a  catalyst.  Romani  pre- 
pared the  metallic  salts  of  these  methylated  mercapto-benzothiazoles  but  not  the  disul- 
fides. 

Mechanism  of  the  Reaction 

Bruni  and  Romani,8  apparently  following  the  lead  of  Jacobson  and 
Frankenbacher,  have  sought  to  explain  the  formation  of  1-mercapto- 
benzothiazole  from  thiocarbanilide,  monophenyl-thio-urea  and  methylene- 
aniline  upon  the  assumption  that  these  substances  first  decompose  to  give 
phenyl  mustard  oil.  They  mention  an  alkali-insoluble  residue,  which 
apparently  was  not  further  investigated.  It  is  shown  in  the  experimental 
part  of  this  paper  that  this  insoluble  residue  consists  chiefly  of  1-anilido- 
benzothiazole  and  that  it  is  formed  in  largest  amounts  when  thiocarban- 
ilide is  used  as  the  starting  material.  Indeed,  it  is  almost  entirely  absent 
when  ammonium  phenyl-dithiocarbamate  or  the  free  aniline  and  carbon 

«  Hofmann,  Ber.,  20,  1788  (1887). 

6  Jacobson  and  Frankenbacher,  Ber.,  24,  1400  (1891). 

6  Rassow,  Dohle  and  Reim,  J.  prakt.  Chem.,  93,  183  (1916). 

7  Bedford  and  Sebrell,  J.  Ind.  Eng.  Chem.,  13,  1034  (1921). 

8  Bruni  and  Romani,  Giorn.  chim.  ind.  applicata,  3,  351  (1921). 


disulfide  are  used.     The  insoluble  residue  obtained  when  the  zinc  salt  is 
used  consists  almost  wholly  of  zinc  sulfide. 

If  the  formation  of  1-mercapto-benzothiazole  from  thiocarbanilide  is 
assumed  to  take  place  through  its  decomposition  products,  such  as  aniline 
and  carbon  disulfide  or  phenyl  mustard  oil,  by  virtue  of  their  reaction  with 
sulfur,  then  little  or  no  1-anilido-benzothiazole  should  be  expected  since 
these  latter  reactions  form  only  small  amounts  of  the  alkali -insoluble  resi- 
due. These  facts  seem  to  indicate  that  direct  sulfurization  of  the  thio- 
carbanilide offers  the  most  satisfactory  explanation  for  the  simultaneous 
formation  of  1-mercapto-benzothiazole  and  1-anilido-benzothiazole.  The 
formation  of  these  two  products  may  be  readily  understood  by  use  of  the 
following  mechanism. 

1.  Thiocarbanilide  may  be  assumed  to  be  in  equilibrium  with   its 
tautomeric   form. 

CeHfiNH— C— NHC6H5^=— C6H6N  =  C— NHC6H5 

S  SH 

2.  This  tautomeric  form  should  occur  as  two  geometric  isomers,  A 
and  B. 

CeHeNH— C—SH  C6H6NH— C— SH 

A  ||  B  || 

N— C6H6  C6H6— N 

3.  When  A  is  sulfurized  and  hydrogen  sulfide  eliminated  1-anilido- 
benzothiazole  is  formed. 

I— C—SH  C6HBNH— C— SH  SR  C6H6NH— C— S 

+S  /—^   — H2S 

— >        ^~\_y  — >      N 

4.  When  B  is  sulfurized  and  aniline  eliminated   1-mercapto-benzo- 
thiazole is  formed. 

[—C—SH  C6H6NH— C— SH  S C—SH 

+S  HS 

II      — ^6-n6iN-n2  ; \    - 

-N 


In  the  experimental  part  of  this  paper  it  is  shown  that  the  combined 
yield  of  these  two  products  will  account  for  between  90  and  96%  of  the 
thiocarbanilide  used.  These  facts  are  of  especial  significance  since  Jacob- 
son  and  Frankenbacher  have  shown  that  1-anilido-benzothiazole  is  not 
formed  by  the  action  of  aniline  on  1-mercapto-benzothiazole. 

The  formation  of  1-mercapto-benzothiazole  from  phenyl-dithiocarbamic 
acid  and  its  salts  is  not  so  easily  traced.  It  may  be  explained  in  one  of 
three  ways. 

1.  The  dithiocarbamic  acid  derivative  may  decompose  with  the  forma- 
tion of  phenyl  mustard  oil,  which  in  turn  reacts  with  the  sulfur  to  form 
the  mercapto-benzothiazole.  This  explanation  would  correspond  to  that 
offered  by  Jacobson  and  Frankenbacher5  and  by  Bruni  and  Romani8  to 


account  for  the  formation  of  the  mercapto-thiazoles  in  other  cases.  The 
former  workers  obtained  a  45%  yield  of  1-mercapto-benzothiazole  by  heat- 
ing phenyl  mustard  oil  with  sulfur.  In  this  Laboratory  as  high  as  60% 
yields  have  been  obtained  by  the  same  method.  By  reference  to  Table  I 
it  will  be  seen  that  phenyl-dithiocarbamic  acid  and  its  salts  yield  75  to 
80%  of  the  same  thiazole.  It  seems  doubtful,  therefore,  whether  phenyl 
mustard  oil  is  an  intermediate  product  in  the  formation  of  mercapto- 
benzothiazoles  from  phenyl-dithiocarbamic  acid  and  its  salts. 

2.  Phenyl-dithiocarbamic  acid  and  its  salts  not  only  yield   phenyl 
mustard  oil  but  also  undergo  a  second  type  of  splitting  with  the  formation 
of  aniline  and  carbon  disulfide.     This  is  especially  true  of  the  ammonium 
salt,  since  thiocarbanilide  is  found  among  its  decomposition  products. 
Thiocarbanilide  should  be  expected  to  react  with  sulfur  yielding  the  mer- 
capto-thiazole  according  to  the  scheme  given  above.     Such  a  reaction 
would  produce  a  large  amount  of  an  alkali-insoluble  residue  which  experi- 
ment shows  is  not  the  case. 

3.  There  is  no  a  priori  reason  why  phenyl-dithiocarbamic  acid  and  its 
salts  should  not  undergo  direct  sulfurization  as  readily  as  its  anilide. 
The  present  authors  believe  this  to  be  a  more  logical  explanation  and  that 
the  mercapto-benzothiazole  is  produced  by  the  loss  of  the  appropriate 
hydrosulfide  from  the  addition  product  thus  formed. 

Experimental  Part 

Preparation  of  Materials. — The  ammonium  salts  of  the  aryl  dithiocarbamic  acids 
were  prepared  by  the  method  of  Losanitch,9  by  the  reaction  of  the  arylamine  and  carbon 
disulfide  in  the  presence  of  ammonium  hydroxide  or  ammonium  sulfide  in  alcoholic  solu- 
tion. The  rapid  decomposition  characteristic  of  these  ammonium  salts  has  caused  many 
investigators  to  doubt  their  actual  existence. 

Ammonium  phenyl-dithiocarbamate  prepared  in  the  presence  of  strong  ammonium 
hydroxide  is  quite  stable  and  after  careful  drying  shows  little  tendency  to  decompose. 
By  recrystallization  from  strong  ammonium  hydroxide  it  is  obtained  as  long  hexagonal 
crystals,  which  when  carefully  washed  and  dried  have  been  kept  for  more  than  a  year 
without  evidence  of  decomposition.  In  the  presence  of  moisture  however  it  is  slowly 
converted  into  thiocarbanilide,  carbon  disulfide  and  ammonia,  as  reported  by  Losanitch. 

These  ammonium  salts  may  also  be  prepared  by  passing  ammonia  gas  into  a  solution 
of  aryl  amine  and  carbon  disulfide  in  benzene,  but  the  method  has  no  advantages  over 
the  one  previously  described. 

The  zinc  salts  of  the  aryl  dithiocarbamic  acids  were  prepared  by  adding  a  solution 
of  zinc  acetate  in  water  or  ammonium  hydroxide  to  an  aqueous  solution  of  the  corre- 
sponding ammonium  salt.  They  may  also  be  prepared  by  the  method  of  Krulla10who 
added  the  metallic  oxide  to  a  mixture  of  the  aniline  and  carbon  disulfide.  The  first 
method  gives  the  purer  product. 

Thiocarbanilide  and  its  substituted  derivatives  were  prepared  by  the  action  of  the 
aryl  amine  with  carbon  disulfide  in  the  usual  manner.  These  products  were  purified  un- 


9  Losanitch,  Ber.,  24,  3022  (1891);  Ann.,  166,  142  (1873). 

10  Krulla,  Ber.,  46,  2669  (1913). 


9 

til  the  melting  points  varied  not  more  than  3°  or  4°  from  the  highest  recorded  values  in 
any  case. 

Apparatus. — The  apparatus  used  in  this  work  consisted  of  a  steel  autoclave  mounted 
in  an  electric  resistance  oven.  The  autoclave,  which  had  a  capacity  of  2  liters,  was 
turned  from  tool  steel  and  was  capable  of  withstanding  a  pressure  of  2000  pounds.  The 
oven  consisted  of  3  heating  elements  specially  wound  from  Chromel  wire  and  mounted  on 
a  reinforced  frame  built  up  from  Alundum  cement.  The  heating  unit  was  effectually 
insulated  by  packing  in  a  mixture  of  asbestos  and  magnesite.  Temperature  control, 
secured  by  an  external  hot  wire  rheostat,  was  sufficiently  exact  to  make  it  possible  to 
reproduce  any  given  set  of  temperature  conditions.  Temperatures  up  to  600°  were 
easily  obtained. 

Procedure. — The  substance  used  to  prepare  the  thiazole  was  mixed 
intimately  with  one  molar  equivalent  of  sulfur  and  placed  in  the  auto- 
clave. Practically  all  runs  were  made  by  bringing  the  oven  to  a  temper- 
ature of  390-400°,  then  lowering  the  autoclave  to  position  and  allow- 
ing the  temperature  of  the  reaction  mixture  to  rise  until  the  pressure 
reached  a  maximum.  The  pressure  would  rise  gradually  with  the  tem- 
perature to  about  170-180°  where  the  increase  became  much  more  rapid 
reaching  a  maximum  at  about  225—250°.  The  pressure  generated  varied 
with  the  substance  used  to  produce  the  thiazole,  being  greatest  with  the 
ammonium  salts  and  least  with  the  disubstituted  thio-ureas.  Reproducing 
the  heating  conditions  did  not  always  give  the  same  yield  of  product. 
Usually  the  autoclave  was  withdrawn  from  the  oven  immediately  after 
the  pressure  reached  the  maximum  to  secure  more  rapid  cooling,  and  when 
it  was  quite  cold  the  accumulation  of  hydrogen  sulfide  was  blown  off  and 
the  autoclave  opened. 

Method  of  Purification. — The  crude  reaction  product  was  removed 
from  the  autoclave  by  solution  in  warm  dil.  sodium  hydroxide  solution. 
This  alkaline  solution  was  submitted  to  steam  distillation  to  remove  any 
free  aryl  amine.  The  solution  was  then  filtered  from  a  residue  of  insoluble 
material  and  fractionally  precipitated  by  the  addition  of  small  portions 
of  hydrochloric  acid.  The  first  precipitates  were  very  dark  and  carried 
down  most  of  the  coloring  matter.  The  last  fractions  precipitated  con- 
sisted of  the  1-mercapto-benzothiazole  in  an  almost  pure  form.  The 
product  was  redissolved  in  sodium  carbonate  solution  and  reprecipitated 
further  purification  varied  depending  upon  the  nature  of  the  product  and 
will  be  described  for  each  individual  substance  together  with  its  physical 
and  chemical  characteristics. 

Discussion  of  the  Experimental  Results 

The  data  collected  in  the  preparation  of  1-mercapto-benzothiazole  and 
6  of  its  substituted  derivatives  are  set  forth  in  Table  I.  Not  all  of  the  runs 
made  are  listed,  but  those  selected  are  typical.  Failure  to  prepare  all 
the  thiazoles  by  each  of  the  four  methods  was  due  to  a  lack  of  sufficient 
quantity  of  the  aryl  amines  from  which  to  prepare  the  necessary  starting 


10 

materials.     In  such  cases  that  method  was  selected  which  promised  to 
yield  the  largest  amount  of  easily  purified  product. 

TABLE  I 
DATA  ON  THE  PREPARATION  OP  I-MERCAPTO-BENZOTHIAZOLES 


Substances 
used 


Max.     Max.  NaOH 

Used  Time  temp,     press,  insol.  Yield 

G.      Hrs.      °C.        Lbs.       G.  G.  % 


Product 


1    ThiocarbanilSde 456     4.00     265       300     105.0     245.0     74. 


2 

Ammonium    phenyl-dithiocarba- 

mate   

263 

1 

50 

224 

1000 

7 

3 

174.5 

74.0 

1-Mercapto- 

8 

Zinc  phenyl-dithiocarbamate.  .  .  . 

350 

1.83 

247 

575 

101 

0 

225.2 

77.5 

benzothiazole 

4 

Aniline  and  carbon  disulfide  

93 

76 

2 

.00 

271 

571 

9 

0 

128.0 

76.  6 

5 

Di-o-tolyl-thio-urea  

512 

4 

.00 

293 

520 

194 

n 

177.0 

49.0 

6 

Ammonium     o-tolyl-dithiocarba- 
matc 

170 

1 

.42 

253 

1050 

18.5 

103.0 

67.0 

1-Mercapto- 

7 

Zinc  o-tolyl-dithiocarbamate  

214 

1 

.25 

266 

625 

137.5 

65.7 

37.0 

3-methyl- 

8 

o-Toluidine  and  carbon  disulfide.  . 

105 

benzothiazole 

76 

1 

.58 

255 

675 

34 

7 

80.4 

45.  5( 

9 

Ammonium    m-tolyl-dithiocarba- 

1  1-Mercapto- 

mate           

186 

1 

.00 

242 

625 

15 

5 

116.6 

69  5  \  4-methyl- 

1  benzothiazole 

10 

Di-/>-tolyl-thio  urea  

384 

1 

.50 

295 

475 

104. 

o 

185.0 

68.0  < 

11 

Ammonium     £-folyl-dithiocarba- 

1-Mercapto- 

mate  

308 

1 

.25 

227 

1050 

22. 

8 

199.2 

VI.  5 

5-methvl- 

12 

/>-Toluidine  and  carbon  disulfide. 

105 

benzothiazole 

76 

1.50 

240 

738 

44. 

0 

102  0 

57.  9  < 

13 

2,4.2',  4'  -  Tetramethyl-diphenyl- 

thio-urea  •  

260 

1 

.25 

228 

450 

178. 

6 

30.7 

17.2 

14 

Ammonium     o,£-xylyl-dithiocar- 

1-Mercapto- 

bamate  

198 

1 

.70 

220 

1000 

37. 

1 

62.1 

34.5 

3,5-dimethyl- 

15 

Zinc  o,/>-xylyl-dithiocarbamate..  . 

239 

1 

.00 

239 

775 

114. 

5 

66.2 

32.4 

benzothiazole 

16 

m-Xylidine  and  carbon  disulfide.  . 

121 

76 

1 

.58 

247 

725 

84. 

0 

59.3 

30.4 

17 

Di-/>-phenety!-thio  ur*a  

269 

1 

.83 

244 

325 

109. 

o 

104.5 

58.4  ' 

1-Ivtercapto- 

18 

Ammonium  p  -  phenetyl  -  dithio- 

5-ethoxy- 

carbamate  

339 

1 

.10 

206 

1275 

8. 

5 

227.5 

73.0 

Hpn  rf  nt  h  i  a  7r»1  f» 

19 

Zinc  p  phenetyl-dithiocarbamate 

288 

1 

58 

246 

550 

125. 

0 

147.7 

59.3  f 

20 

£-Phenetidine  and  carbon  disul- 

137 

fide  

76 

1 

.30 

249 

455 

49. 

o 

117.4 

55.6 

21 

0-Anisidine  and  carbon  disulfide. 

62 

1-Mercapto- 

38 

1 

.20 

237 

365 

36. 

5 

61.5 

62.0 

5-methoxy- 

Runs  1  and  5  were  made  first,  and  without  preheating  the  oven,  thus 
accounting  for  the  longer  reaction  time.  In  later  runs  the  reaction  time 
was  shortened  to  avoid  heat  decomposition.  The  ammonium  aryl-dithio- 
carbamates  gave  in  general  the  best  yields,  but  the  products  from  the  zinc 
salts  contained  less  tarry  material  and  were  therefore  more  readily  purified. 

ThQ  more  highly  substituted  the  arylamine,  the  lower  the  yield  of  the 
thiazole  so  that  only  a  34.5%  yield  of  the  dimethyl  derivative  was  obtained 
even  from  the  ammonium  salt  of  xylyl-dithiocarbamic  acid. 

The  disubstituted  thio-ureas  always  yielded  large  amounts  of  an  alkali- 
insoluble  by-product.  The  yield  of  •  this  substance  is  markedly  lower 
when  the  ammonium  salt  of  the  aryl-dithiocarbamic  acid  or  the  aryl  amine 


11 

and  carbon  disulfide  were  used.     In  the  case  of  the  zinc  salts  the  alkali- 
insoluble  product  consisted  almost  wholly  of  zinc  sulfide. 

Nature  of  the  Alkali-Insoluble  Material 

In  the  case,  of  thiocarbanilide  the  alkali-insoluble  part  of  the  reaction 
product  was  found  to  consist  largely  of  1-anilido-benzothiazole.  The 
purification  of  this  anilido  derivative  proved  to  be  so  tedious  that  the  sep- 
aration of  a  similar  derivative  was  not  successfully  completed  in  any  of 
the  other  cases. 

1-Anilidobenzothiazole.4 — The  alkali-insoluble  residue  from  the  thiocarbanilide- 
sulfur  reaction  mixture  was  dissolved  in  alcohol  and  the  solution  poured  into  dil.  hydro- 
chloric acid.  A  small  amount  of  acid-insoluble  substance  was  removed  by  filtration  and 
the  anilido  derivative  precipitated  by  the  addition  of  sodium  hydroxide.  Fifty  g.  of  the 
original  residue  gave  36  g.  of  the  purified  anilido  derivative  and  5  g.  of  the  acid-insoluble 
substance,  which  proved  to  be  mostly  1-mercapto-benzothiazole.  After  recrystallization 
from  benzene  the  1-anilido-benzothiazole  was  obtained  as  light  yellow  crystals  melting 
at  154°.  A  mixture  of  this  product  with  1-anilido-benzothiazole  prepared  by  the  action 
of  phenyl  mustard  oil  on  azobenzene  gave  the  same  melting  point. 

Analyses.     Subs.,  0.5470:    46.9  cc.  0.1  N  H2SO4.     Subs.,  0.5102:    BaSO4,  0.5293 
Calc.  for  Ci3H10N2S:   N,  12.38;  S,  14.16.     Found:   N,  12.00;  S,  14.25. 

Using  the  above  data  the  calculated  yield  of  pure  1-anilido-benzothiazole  available 
from  the  105  g.  of  alkali-insoluble  residue  from  the  thiocarbanilide  in  Table  I  is  75.6  g. 
or  16.5%.  By  combining  this  yield  with  the  73.3%  of  1-mercapto-benzothiazole  ob- 
tained, approximately  90.0%  of  the  thiocarbanilide  is  accounted  for.  When  one  con- 
siders the  losses  incident  to  purification  it  is  easy  to  conceive  that  the  actual  yield  of  the 
two  derivatives  is  much  higher,  being  about  96%  on  the  basis  of  the  crude  products. 

Disulfides 

The  disulfides  of  each  of  the  mercapto-thiazoles  described  were  prepared 
by  the  method  of  Hofmann.5  An  alcoholic  or  alkaline  solution  of  the 
mercapto-thiazole  was  oxidized  by  the  gradual  addition  of  an  alcoholic 
solution  of  iodine.  The  resulting  disulfide,  insoluble  in  alcohol  or  alkalies, 
precipitated  immediately,  and  after  it  was  filtered,  washed  and  dried  was 
purified  by  recrystallization  from  the  solvent  indicated  in  each  case. 

Zinc  and  Lead  Salts 

The  zinc  salts  of  the  mercapto-benzothiazoles  may  be  prepared  by  either 
of  two  methods.  A  solution  of  the  ammonium  salt  may  be  precipitated 
by  the  addition  of  a  solution  of  ammonium  zincate,  or  an  alcoholic  solution 
of  the  thiazole  may  be  precipitated  by  adding  an  aqueous  solution  of  any 
zinc  salt.  The  first  method  gives  the  purer  product. 

Both  the  normal  and  the  basic  lead  salts  may  be  prepared,  depending 
upon  the  method  used.  The  normal  lead  salts  were  obtained  by  precipi- 
tating an  alcoholic  solution  of  the  free  thiazole  or  an  aqueous  solution  of 
its  sodium  salt  by  an  aqueous  solution  of  any  soluble  lead  salt.  The  basic 
lead  salts  were  obtained  by  precipitating  an  alkaline  solution  of  the  mer- 


12 

capto-benzothiazole  by  a  solution  of  lead  hydroxide  in  an  excess  of  sodium 
hydroxide.  These  salts  were  thoroughly  washed,  dried  and  subjected 
to  analysis  without  further  purification. 

The  zinc  and  lead  salts  of  each  of  the  several  mercapto-benzothiazoles 
were  prepared,  but  in  only  three  cases  were  they  actually  analyzed  and 
tested  for  their  accelerating  power  as  indicated  in  the  following  pages. 

1-Mercapto-benzothiazole5  is  soluble  in  alcohol,  benzene  and  acetic  acid.  By  re- 
crystallization  from  dil.  alcohol  it  was  obtained  as  light  yellow  needles,  melting  at  177°. 

Analyses.  Subs.,  0.7502:  44.8  cc.  of  0.1  NH2SO4.  Subs.,  0.1016:  BaSO4,  0.2840. 
Calc.  forC7H5NS2:  N,  8.38;  S,  38.32.  Found:  N,  8.37;  S,  38.38. 

THE  BISULFIDE*;  an  amorphous  slightly  yellow  powder,  melting  at  176  °;  yield,  87%. 

THE  ZINC  SAI/T  ;  a  white,  amorphous  powder. 

Analysis.     Calc.  for  Ci4H8N2S4Zn:  N,  7.05;  Zn,  16.45.     Found:  N,  7.21 ;  Zn,  16.05. 

THE  NORMAL  LEAD  SALT;  a  bright  yellow  powder. 

Analysis.     Calc.  for  Ci4H8N2S4Pb :   Pb,  38.40.     Found:  38.49. 

THE  BASIC  LEAD  SALT;  an  amorphous  white  powder. 

Analysis.     Calc.  for  C7H5NS2OPb:   Pb,  53.05.     Found:   52.35. 

l-Mercapto-3-methyl-benzothiazole3b  after  repeated  recrystallizations  from  dil. 
alcohol  and  finally  from  50%  acetic  acid  was  obtained  as  white  needles  melting  at  186°. 

Analyses.  Subs.,  0.7205:  41.2  cc.  of  0.1  NH2SO4.  Subs.,  0.1060,  0.1143:  BaSO4, 
0.2720;  0.2929.  Calc.  for  C8H7NS2:  N,  7.73;  S,  35.36.  Found:  N,  8.00;  S,  35.26, 
35.21. 

THE  BISULFIDE;  white  needles  from  chloroform;  m.  p.,  162°. 

Analysis.     Calc.  for  Ci6H12N2S4:  S,  35.55.     Found:  35.63,  35.48. 

THE  ZINC  SALT;  a  white  amorphous  powder. 

Analysis.     Calc.  for  Ci6Hi2N2S4Zn:  Zn,  15.37.     Found:   15.35. 

THE  NORMAL  LEAD  SALT;  an  amorphous  yellow  powder. 

Analysis.     Calc.  for  Ci6H12N2S4Pb :  Pb,  36.50.     Found:  36.78. 

THE  BASIC  LEAD  SALT;  an  amorphous  white  powder. 

Analysis.     Calc.  for  C8H7NS2OPb:   Pb,  51.24.     Found:   51.50. 

l-Mercapto-4-methyl-benzothiazole3b  was  prepared  only  through  the  ammonium 
salt  of  w-tolyl-dithiocarbamic  acid.  After  successive  recrystallizations  from  75% 
alcohol,  50%  acetic  acid  and  benzene,  it  was  obtained  as  light  yellow  plates  melting  at 
163°. 

Analyses.  Subs.,  0.1585,  0.2270:  BaSO4,  0.4075,  0.5878.  Calc.  for  CsHzNSa: 
3,35.36.  Found:  35.30,35.52. 

THE  BISULFIDE;  white  plates  from  benzene;  m.  p.,  195°. 

Analysis.     Calc.  for  Ci6H12N2S4:  S,  35.55.     Found:  35.57;  35.65. 

l-Mercapto-5-methyl-benzothiazole3b  was  recrystallized  twice  from  50%  acetic 
acid  and  finally  from  benzene.  It  forms  fine,  very  light  yellow  crystals  melting  at  181  °. 

Analyses.  Subs.,  0.7234:  38.37  cc.  of  0.1  N  H2SO4.  Subs.,  0.1860,  0.1985:  Ba- 
SO4,  0.4784,  0.5109.  Calc.  for  C8H7NS2:  N,  7.73;  S,  35.36.  Found:  N,  7.44;  S,  35.39, 
35.34. 

THE  BISULFIDE;  This  was  insoluble  in  alcohol  but  soluble  in  benzene  and  chloro- 
form, white  needles  from  chloroform  melting  at  201-202°. 


13 

Analysis.     Calc.  for  C:6Hi2N2S4:    S,  35.55.     Found:    35.59. 

THE  ZINC  SAI/T;  a  white  amorphous  powder. 

Analysis.     Calc.  for  Ci6Hi2N2S4Zn :   Zn,  15.37.     Found:    15.40. 

l-Mercapto-3,5-dimethyl-benzothiazole  was  recrystallized  several  times  from  al- 
cohol from  which  it  separates  in  light  yellow  crystals  melting  at  250.5°.  It  is  only  very 
slightly  soluble  in  acetic  acid  or  benzene. 

Analyses.  Subs.,  0.2050,  0.1888:  BaSO4,  0.4904,  0.4523.  Calc.  for  C9H9NS2: 
8,32.82.  Found:  32.85,  32.90. 

THE  DisuiyFiDE;  fine  white  needles,  m.  p.  193°,  obtained  by  repeated  precipitation 
from  chloroform  with  the  gradual  addition  of  alcohol. 

Analyses.     Calc.  for  Ci8H16N2S4:  S,  32.99.     Found:  33.02,  33.13. 

l-Mercapto-5-ethoxy-benzothiazole  was  prepared  by  each  of  the  four  methods  but 
only  the  ammonium  ^-phenetyl-dithiocarbamate  yielded  an  easily  purified  product. 
Recrystallized  from  75%  alcohol  and  twice  from  benzene  it  was  obtained  as  well-formed, 
long,  cream-colored  needles  melting  at  198°.  It  is  only  slightly  soluble  in  benzene  and 
other  organic  solvents. 

Analyses.  Subs.,  0.1950,  0.1852:  BaSO4,  0.4303,  0.4044.  Calc.  for  C9H9NOS2: 
8,30.33.  Found:  30.31,29.99. 

Attempts  to  prepare  the  disulfide  were  unsuccessful. 

l-Mercapto-5-methoxy-benzothiazole  was  prepared  by  each  of  the  four  methods  but 
the  product  in  all  cases  proved  very  difficult  to  purify.  It  was  found  that  by  rapidly 
heating  the  reaction  mixture  and  rapidly  cooling  the  autoclave  a  more  easily  purified 
product  was  obtained.  Recrystallized  from  70%  alcohol  and  twice  from  benzene  it 
forms  light  yellow  needles  melting  at  201  °. 

Analysis.  Subs.,  0.1505:  BaSO4,  0.3546.  Calc.  for  C8H7NOS2:  8,32.48.  Found: 
32.37. 

Attempts  to  prepare  substituted  mercapto-benzothiazoles  from  0-anisidine,  the 
o-  and  p-chloro-  and  bromo-anilines  and  ^-aminophenol  were  unsuccessful.  Benzidine 
and  carbon  disulfide  yielded  an  alkali-soluble  product  melting  above  250  °,  which  was  not 
further  investigated. 

Summary  and  Conclusions 

1.  The  chemistry  of  the  zinc  and  ammonium  salts  of  phenyl-dithio- 
carbamic  acid  has  been  extended  and  the  work  of  Losanitch  verified. 

2.  Three  new  methods  for  the  preparation  of  1-mercapto-benzothiazole 
and  its  substituted  derivatives  have  been  described. 

3.  A  fourth  method  previously  described  by  one  of  us  and  independ- 
ently announced  about  the  same  time  by  Bruni  of  Italy,  has  been  extended 
to  these  derivatives. 

4.  The  mechanism  of  the  reaction  involved  in  the  formation  of  1- 
mercapto-benzothiazoles  by  the  action  of  sulfur  on  disubstituted  thio- 
ureas  is  fully  discussed.     Experimental  evidence  is  offered  in  support  of 
the  view  that  this  reaction  takes  place  by  virtue  of  direct  sulfurization 
of  the  cis-mercapto  form  of  thiocarbani^de  and  subsequent  loss  of  the 
aryl  amine  to  form  the  mercapto-benzothiazole.     It  is  also  pointed  out  that 


14 

sulfurization  of  the  Zraws-mercapto  form  of  thiocarbanilide  and  loss  of  hy- 
drogen sulfide  explains  the  simultaneous  formation  of  anilido-benzothiazole. 

5.  Direct  sulfurization  and  subsequent  elimination  of  the  correspond- 
ing hydrosulfide  is  offered  as  the  best  explanation  of  the  formation  of 
mercapto-benzothiazoles  by  the  action  of  sulfur  on  the  aryl  dithiocarbamic 
acids  and  their  salts. 

6.  Six    substituted    mercapto-benzothiazoles    are    described    together 
with  four  of  the  corresponding  disulfides. 

7.  Methods  are  given  for  the  preparation  of  the  zinc,  normal  lead  and 
basic  lead  salts  of  the  1 -mercapto-benzothiazoles  and  in  three  cases  such 
salts  are  described. 


PART  II 

1 — Mercaptobenzothiazole 
and  Its  Derivatives  as  Ac- 
celerators of  Rubber 
Vulcanization12 

THE  use  of  1-mercaptobenzothiazole  as  an  accelerator 
of  vulcanization  was  first  suggested  by  Bedford  and 
Sebrell. 3  This  announcement  was  closely  followed  by 
that  of  Bruni  and  Romani,  *  who  set  forth  in  detail  a  method 
for  the  preparation  of  1-mercaptobenzothiazole  by  heating 
thiocarbanilide  with  sulfur  under  pressure.  They  also  pro- 
posed a  theory  for  the  mechanism  of  the  accelerating  action 
as  applied  to  thiocarbanilide  and  the  thiazole  derivatives. 

The  second  paper  of  Bedford  and  Sebrell5  revealed  that  the 
method  of  preparation  described  by  Bruni  and  Romani  had 
previously  been  known  to  them,  and  pointed  out  that  the 
mechanism  used  by  the  latter  workers  to  explain  the  action 
of  thiocarbanilide  as  an  accelerator  was  untenable. 

The  present  investigation  is  a  continuation  of  the  work  of 
Bedford  and  Sebrell.  It  has  been  carried  out  with  the  follow- 
ing purposes  in  mind: 

1 — To  make  a  comparative  study  of  the  relative  value  of 
several  derivatives  of  1-mercaptobenzothiazole  as  accelerators  of 
vulcanization. 

2 — By  a  process  of  substitution  and  elimination  to  determine 
what  part  of  the  mercaptobenzothiazole  structure  is  responsible 
for  the  accelerating  action  of  these  compounds. 

PREPARATION  OF  MATERIALS 

I-MERCAPTOBENZOTHIAZOLE  AND  ITS  DERIVATIVES — The 
preparation  and  properties  of  1-mercaptobenzothiazole  and 
its  3-methyl,  4-methyl,  5-methyl,  3,5-dimethyl,  5-methoxy, 
and  5-ethoxy  derivatives  have  been  fully  described  in  Part  I. 

OTHER  BENZOTHIAZOLE  DERIVATIVES — In  order  to  deter- 
mine the  particular  grouping  in  the  1-mercaptobenzothiazole 
structure  responsible  for  the  accelerating  action,  it  became 
necessary  to  prepare  a  series  of  related  benzothiazole  deriva- 
tives. In  each  case  these  compounds  differed  from  the  true 
1-mercaptobenzothiazole  by  a  single  atom  or  grouping,  the 
remaining  part  of  the  molecular  structure  being  identical. 
All  these  substances  have  been  previously  described,  but  the 
details  of  their  preparation  are  incomplete. 

1  Presented  before  the  Division  of  Rubber  Chemistry  at  the  64th  Meet- 
ing of  the  American  Chemical  Society,  Pittsburgh,  Pa.,  September  4  to  8, 
1922. 

2  J.  Ind.  Eng.  Chem.,  15,  1009. 
s  Ibid.,  13,  1034  (1921). 

<  Giorn.  chtm.  ind.  applicaia,  3,  196  (1921). 
«  Ind.  Eng.  Cbem.,  14,  25  (1922). 


16 


1  -Hydroxybenzothiazole, f 


/s\ 
C6H/      ^COH, 


was  prepared 


by  the  hydrolysis  of  1-chlorobenzothiazole,  according  to  the 
method  described  by  Hofmann.6  Long  heating  of  the  free 
1-chlorobenzothiazole  does  not  accomplish  the  hydrolysis. 
However,  if  the  hydrogen  chloride  addition  product  of  1- 
chlorobenzothiazole  is  heated  with  alcohol  for  40  hours,  the 
hydrolysis  is  almost  complete.  The  solution  was  rendered 
strongly  acid  and  precipitated  by  dilution  with  water.  After 
two  recrystallizations  from  75  per  cent  alcohol  1-hydroxy- 
benzothiazole  was  obtained  as  white  needles  melting  at  136° 
C.  This  is  the  same  melting  point  as  given  by  Hofmann. 


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The  hydrolysis  of  the  hydrochloride  may  be  accomplished  in  a 
much  shorter  time  by  heating  with  alcohol  under  pressure. 


1-Amidobenzothiazole,    C6H4 


—  NH2,    was    first    pre- 


pared  by  Hofmann7  by  the  action  of  alcoholic  ammonia  on 
1-chlorobenzothiazole  at  160°  C.  In  this  laboratory  the 
method,  as  described,  always  yields  the  product  in  the  form  of 
a  noncrystallizable  sirup.  Excellent  results  were  obtained  by 
the  following  method  : 

Fifteen  grams  of  the  1-chlorobenzothiazole  together  with  45  cc. 
of  saturated  alcoholic  ammonia  and  15  cc.  of  aqueous  ammonia 
were  sealed  in  a  tube  and  heated  at  200°  C.  for  4  hours.  On 
opening  the  tube  the  contents  were  mixed  with  50  per  cent  alco- 
hol, strongly  cooled  in  an  ice  bath  and  diluted  very  slowly  with 
water  to  1  liter.  After  standing  in  an  ice  box  for  12  hours  1- 
amidobenzothiazole  was  deposited  as  fine  white  needles,  with 
only  a  trace  of  the  sirupy  impurity.  The  yield  was  10  grams. 
This  product  after  recrystallization  from  benzene  melted  at  127  ° 
C.,  compared  with  129°  C.  recorded  by  Hofmann. 

6  Ber.,  12,  1126  (1879);  13,  9  (1880). 

»  Ibid.,  13,  11  (1880). 


17 


/Ov 

1-Mercaptobenzoxazole,   C6H4(       ^CSH,  was  prepared  by 


refluxing  an  alcoholic  solution  of  o-aminophenol  with  carbon 
disulfide  according  to  the  method  of  Diinner.8  After  the  pre- 
liminary purification  by  precipitating  from  sodium  carbonate 
solution,  and  recrystallization  from  water,  the  product  melted 
at  193°  C.,  the  same  as  recorded  by  Diinner. 

CH2—  S\ 
H-MercaptotMazolen,      I  J^CSH,    was    prepared    for 

the  purpose  of  comparison  with  1-mercaptobenzothiazole  to 
determine  the  effect  of  the  aromatic  nucleus  upon  the  ac- 
celerating power  of  the  mercaptothiazoles.  The  compound 
was  first  described  by  Gabriel9  who  prepared  it  by  the  action 
of  j8-bromoethylamine  hydrochloride  and  carbon  disulfide  in 
alkaline  solution.  Some  difficulty  was  experienced  in  the 
isolation  of  both  the  hydrobromide  and  its  reaction  product 
with  carbon  disulfide,  but  a  sufficient  quantity  was  finally 
obtained  to  determine  its  relative  value  as  an  accelerator. 

RESULTS  OF  COMPOUNDING  TESTS 

All  the  compounds  listed  above  were  tested  to  ascertain 
their  relative  value  as  accelerators  of  vulcanization.  The 
following  experimental  formula  was  used: 

100  .00  parts  of  rubber  (smoked  sheet) 
5  .00  parts  of  zinc  oxide 
3.50  parts  of  sulfur 

1  .00  part  of  1-mercaptobenzothiazole  or  a  molecular  equiva- 
lent of  its  derivative  or  analog 

With  a  few  exceptions,  each  of  these  compounds  produced 
very  rapid  curing  in  the  formula  given. 

Since  many  of  these  substances  are  rapid  and  powerful 
accelerators,  slight  variations  in  the  milling  and  curing  may 
produce  differences  of  some  magnitude  in  the  tensile-time 
diagrams.  For  this  reason  the  conditions  of  milling  and  cur- 
ing were  standardized  as  follows  : 

The  stocks  were  all  mixed  in  1-kg.  batches  on  a  small  experi- 
mental mill.  After  milling  for  20  minutes  to  break  down  the 
rubber,  the  zinc  oxide  and  accelerator  were  added  and  mixed 
thoroughly  into  the  stock  for  10  minutes.  The  stock  was  then 
cooled  to  the  lowest  workable  temperature  and  the  sulfur  added. 
After  5  minutes  further  mixing  it  was  removed  from  the  mill. 
All  samples  were  milled  as  nearly  as  possible  as  described  above. 

The  curing  was  conducted  in  a  steam  platen  press  in  which  the 
temperature  was  controlled  as  closely  as  possible.  All  cures 
were  made  in  the  same  set  of  molds,  which  had  been  preheated 
before  use  by  allowing  them  to  remain  for  at  least  30  minutes  in 
the  press  at  the  required  temperature.  All  the  cures  were  made 
using  the  same  decks  of  the  same  press.  The  vulcanized  sheets 
after  being  removed  from  the  molds  were  plunged  into  a  tank  of 
cold  water  to  stop  vulcanization. 

s  Ber.,  9,  465  (1876);  16,  1825  (1883). 
oibid.,  21,  566  (1888);  22,  1137,  1152  (1889). 


18 


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20 


The  physical  data  given  in  the  following  tables  were  ob- 
tained by  two  observers  on  a  Scott  testing  machine  using 
dumb-bell  test  pieces.  The  average  of  two  or  more  closely 
agreeing  strips  is  given  for  each  cure. 

The  results  of  the  physical  tests  on  the  cures  made  with  1- 
mercaptobenzothiazole,  its  alkyl  and  alkyloxy  derivatives, 
are  recorded  in  Table  I,  and  are  represented  graphically  in 
Figs.  1  and  2.  In  comparing  the  relative  activity  of  these 
compounds,  the  writers  have  used  as  a  basis  of  comparison 
the  time  in  which  each  compound  was  judged  to  give  the  best 
technical  cure.  This  can  best  be  determined  by  subjecting 
the  stocks  themselves  to  certain  arbitrary  tests,  such  as  the 
effect  of  repeated  flexing  and  resistance  to  tear.  The  nature 
of  the  grain  as  indicated  by  the  appearance  of  the  tear  is  also 


taken  into  consideration.  The  results  of  such  tests  are  given 
for  each  cure  in  the  tables  in  the  column  "State  of  Cure." 
In  some  cases  the  cures  were  not  sufficiently  close  so  that,  for 
example,  a  5-minute  cure  might  be  judged  by  means  of  these 
tests  to  be  undercured,  while  a  10-minute  cure  would  be  con- 
siderably overcured.  In  such  cases  the  writers  have  inter- 
polated for  the  time  of  best  technical  cure. 

The  relative  order  of  activity  of  these  compounds  as  ac- 
celerators cannot  well  be  determined  from  a  comparison  of 
the  maximum  tensiles  obtained,  since  the  test  sheet  which 
would  give  the  highest  tensile  is  in  almost  all  cases  overcured 
from  a  practical  standpoint.  Any  attempt  to  classify  the 
compounds  on  the  basis  of  the  so-called  optimum  cure,  as 
given  by  either  the  maximum  tensile  product  or  the  energy  of 
resilience,  or  a  combination  of  these  factors,  would  be  sub- 
ject to  the  same  objection  as  given  above  for  the  classification 
in  order  of  maximum  tensile. 


21 

If  it  is  desired  to  utilize  the  stress-strain  data  in  determining 
the  relative  activity  of  these  compounds,  they  can,  for  ex- 
ample, be  ranked  in  the  order  of  the  highest  tensile  stress  at 
700  per  cent  elongation  given  by  a  10-minute  cure  on  each 
compound  at  40  pounds  steam  pressure.  In  this  way  the  er- 
rors inherent  in  a  determination  of  the  maximum  tensile 
are  avoided.  If  such  a  classification  is  made  it  will  be  found 
that  the  compounds  arrange  themselves  in  exactly  the  same 
order  as  determined  by  the  best  technical  cure. 

The  writers  are  therefore  inclined  to  classify  the  various 
mercaptobenzothiazoles  according  to  the  time  necessary 
to  produce  the  best  cure  from  a  practical  standpoint  as  shown 
by  the  above  arbitrary  tests,  and  to  use  the  stress-strain  data 
as  a  measure  of  the  quality  of  the  stocks. 

It  is  realized  that  the  present  method  of  determining  the 
relative  activity  might  not  seem  satisfactory  to  all,  and 
for  this  reason  the  complete  data  for  the  physical  tests,  to- 
gether with  the  tensile  product  and  the  energy  of  resilience, 
have  been  included  in  the  tables. 

Since  only  one  formula  was  used  in  testing  all  these  acceler- 
ators and  because  in  some  cases  an  insufficient  number  of 
cures  have  been  obtained,  it  is  obvious  that  the  results  herein 
presented  cannot  be  considered  as  representing  a  complete 
or  detailed  compounding  investigation.  They  are,  however, 
considered  satisfactory  from  the  standpoint  of  determining 
the  relative  activity  of  the  various  mercaptobenzothiazoles 
and  their  derivatives  as  accelerators. 

Fig.  1  shows  the  tests  made  on  cures  at  40  pounds  steam 
pressure,  while  Fig.  2  gives  the  same  data  for  cures  at  20 
pounds  steam  pressure.  The  relative  activity  of  the  1-mer- 
captobenzothiazoles  and  the  time  required  to  produce  the  best 
technical  cure  are  given  in  Table  II. 

TABLE  II— ORDER  OF  REACTIVITY  OF  THE  I-MERCAPTOBENZOTHIAZOLES  AS 

ACCELERATORS 
<— 40  Pounds  Pressure — -  / — 20  Pounds  Pressure — - 

Time  of  Time  of 

Best  Tech-    Order  of     Best  Tech-       Order  of 

SUBSTANCE  nical  Cure      Activity     nical  Cure       Activity 

1  -  Mercapto  -  3  -  methyl- 

benzothiazole 5  min.  1  18  min.  1 

1  -  Mercapto  -  3,5  -  di- 

methylbenzothiazole. . .  8  min.  2  25  min.  2 

1-Mercaptobenzothiazole.          10  min.  3  30  min.  3 

1  -  Mercapto  -  5  -  methyl- 

benzothiazole 18  min.  4  55  min.  4 

1  -  Mercapto  -  5  -  ethoxy- 

benzothiazole 20  min.  5  60  min.  5 

1  -  Mercapto  -  5-methoxy- 

benzothiazole 1  hr.  6  2  hrs.  6 

The  data  showing  the  relative  curing  power  of  1-mercap- 
to-4-methylbenzothiazole  are  not  given  in  Table  II.  A 
limited  supply  of  this  material  prevented  it  from  being  in- 
cluded when  the  foregoing  tests  were  made.  It  had  pre- 
viously been  tested  in  a  different  formula.  Under  these  con- 
ditions its  activity  as  an  accelerator  was  found  to  lie  between 
the  5-methyl  and  3-methyl  derivatives,  being  very  close  to 
that  of  1-mercaptobenzothiazole. 


22 


In  the  absence  of  zinc  oxide  the  1-mercaptobenzothiazoles 
are  without  appreciable  accelerating  action.  Compounded  in 
the  foregoing  formula,  except  that  the  zinc  oxide  was  omitted, 
1-mercaptobenzothiazole  cured  at  40  pounds  steam  pressure 
for  2  hours  gave  a  maximum  tensile  strength  of  96  kg.  per 
sq.  cm.  Curing  the  same  mixture  at  20  pounds  steam  pres- 
sure for  the  same  time  gave  a  maximum  tensile  strength  of 
50  kg.  per  sq.  cm.  These  substances,  therefore,  have  no  prac- 
tical value  as  accelerators  except  in  the  presence  of  zinc  or 
lead  oxides.  From  these  results  it  seems  probable  that  in  the 
process  of  vulcanization  the  mercaptobenzothiazoles  are  con- 
verted into  the  corresponding  zinc  salts,  these  salts  acting  as 
the  true  accelerating  agents.  To  test  this  assumption  the  zinc 
and  lead  salts  of  1-mercapto-,  l-mercapto-3-methyl,  and  1- 
mercapto-5-methylbenzothiazoles  were  tested  in  the  foregoing 


c,,o 


riGURC  NO  3 

HA*.IMUM  TENSIL  ZO'ST 

ZINC  SAL  T    Of  - 

tt-inERc*PToaenzo  THIAIOLC 

0-2         •         &ritTHVI.BfN20TH 
C-J        •         5HCTHVL 


Tine  or  CURE 


formula.  The  results  are  shown  in  Tables  Ilia  and  III6  and 
the  tensile- time  curves  in  Figs.  3  and  4. 

The  zinc  salt  of  the  5-methyl  derivative  is  slightly  faster 
in  its  curing  action  than  the  zinc  salt  of  the  free  1-mercapto- 
benzothiazole or  its  3-methyl  derivative.  The  last  two  are 
about  equal  in  curing  power.  On  the  other  hand,  the  lead 
salt  of  the  3-methyl  derivative  is  the  most  active  of  the 
three  lead  salts,  giving  good  cures  in  15  minutes  at  20  pounds 
steam  pressure.  The  two  remaining  lead  salts  are  about  equal 
in  activity  but  vary  in  the  tensile  strengths  produced. 

In  all  cases  the  zinc  and  lead  salts  of  these  mercaptobenzo- 
thiazoles are  much  more  powerful  accelerators  than  the  corre- 
sponding free  compounds.  The  zinc  salt  is,  generally,  more 
powerful  than  the  corresponding  lead  salt.  These  metallic 
salts  must  also  be  used  with  additional  metallic  oxide  to  obtain 
their  maximum  accelerating  power. 


23 


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24 


The  disulfides  of  the  several  mercaptobenzothiazoles  were 
also  tested  in  the  foregoing  formula,  and  were  found  to  be 
considerably  less  active  than  the  free  thiazoles. 

Two  conclusions  may  be  drawn  from  these  facts: 

1 — The  1 -mercaptobenzothiazoles  when  used  as  accelerators 
first  form  the  metallic  salts  by  action  with  the  metallic  oxide 
present.  These  salts  are  the  active  accelerators. 

2 — The  salts  so  formed  tend  to  decompose  during  the  process 
of  vulcanization  and  an  excess  of  the  metallic  oxide  must  be  pres- 
ent to  reform  the  salts  and  thus  maintain  the  accelerating  action 

TESTS  ON  COMPOUNDS  SIMILAR  TO  I-MERCAPTOBENZO- 
THIAZOLE 

ju-Mercaptothiazoline  (A),  1-mercaptobenzoxazole  (B), 
1-amidobenzothiazole  (C),  and  1-hydroxybenzothiazole  (D) 
were  also  tested  for  their  accelerating  action  Each  of  these 


|J 


MAXIMUM  TENSIL  10'STPH 

A  MCRCAPTO    TKIAZOLIN 

B      1-MCKCAPTO  BCNZ.OXAZOLC 
C      l-Af1IOOBCNZOTHlAZOI.£. 
a     1-HYOROXYOCNZOTHmZOLE. 


TIME  or  CURE 


compounds  resembles  1-mercaptobenzothiazole  in  certain 
parts  of  its  structure.  The  results  of  these  tests  are  recorded 
in  Table  IV  and  shown  graphically  in  Fig.  5. 


l-MERCAPTOBENZOTHIAZOLE 


CSH 


COH 


(D) 


The  results  show  that  the  /x-mercaptothiazoline  exerts  a 
marked  accelerating  action,  but  much  less  than  that  of  1- 


25 


mercaptobenzothiazole.  This  would  seem  to  indicate  that 
the  accelerating  power  is  invested  in  the  mercaptothiazole 
group.  When  the  sulfur  atom  of  the  thiazole  ring  is  replaced 
by  oxygen  as  in  1-mercaptobenzoxazole,  the  accelerating 
action  is  still  evident  but  much  lower  than  that  of  either  1- 
mercaptobenzothiazole  or  ju-mercaptothiazoline. 

If  tne  mercapto  group  is  replaced  by  an  amino  or  an  hy- 
droxyl  group,  as  in  1-amidobenzothiazole  or  1-hydroxybenzo- 
thiazole,  respectively,  the  accelerating  action  is  very  greatly 


FIGURE  NO 
TfNSIL 


A-  1  nCRCAPTO  BEfiUOr 
B-l  •       3r}£THYL 

C-l  -        SHfTHYt. 


TIMC  OF  CUftt 


diminished;  indeed,  in  the  latter  substance  it  is  almost  entirely 
absent. 

The  conclusions  to  be  drawn  from  these  results  are  as  fol- 
lows: 

1  —  The  accelerating  action  of  1  -mercaptobenzothiazole  and  its 
derivatives  is  invested  primarily  in  the  mercaptothiazole  group. 
The  presence  of  the  benzene  nucleus  adds  markedly  to  the  ac- 
celerating value.  Whether  this  is  due  to  the  attendant  increase 
in  the  molecular  weight  or  more  directly  concerned  with  the 
chemical  characteristics  of  the  benzene  nucleus,  is  not  yet  deter- 
mined. 


2  —  The  atomic  grouping 


CSH,   is  directly    responsible 


for  the  accelerating  action.  Any  change  in  this  grouping  either 
greatly  diminishes  or  completely  destroys  the  accelerating  value. 
3  —  The  mercapto  group  is  more  essential  to  the  acceleration 
than  the  sulfur  of  the  thiazole  ring,  although  both  are  necessary 
to  the  best  results. 

MECHANISM  OF  ACCELERATION  BY  MERCAPTOBENZOTHIAZOLE 
DERIVATIVES 

Several  theories  have  recently  been  advanced  to  explain 
the  vulcanization  of  rubber  by  organic  compounds  containing 


26 

the  mercapto  group.  Bruni  and  Romani5  have  proposed  the 
following  mechanism  for  the  acceleration  of  vulcanization  by 
mercaptobenzothiazoles : 

2R— SH  +  ZnO >  (R— S)2Zn  +  H2O 

x(R— S)2Zn  +  Sx >  xR— S— S— R  +  xZnS 

R— S— S— R >  R— S— R  +  S  (active) 

They  have  lately  extended  this  theory  to  many  other  well- 
known  accelerators,  such  as  thiocarbanilide  and  aldehyde 
ammonia.11  It  is  chiefly  interesting  as  it  applies  to  mercapto- 
benzothiazole  derivatives.  The  disulfides  are  less  active 
accelerators  than  the  free  mercaptobenzothiazoles,  both  with 
and  without  the  presence  of  zinc  oxide.  In  the  absence  of 
zinc  oxide  both  types  of  derivatives  are  almost  without  ac- 
celerating power.  These  facts  seem  to  show  that  the  disulfide 
cannot  be  the  active  agent  in  this  type  of  acceleration.  The 
theory  is  therefore  inadequate  and  cannot  be  accepted. 

Bedford  and  Sebrell6  have  proposed  a  theory  to  account 
for  the  action  of  mercapto  compounds  as  accelerators,  which 
may  be  stated  as  follows : 

2R— SH  +  ZnO >  (R— S)2Zn  +  H2O 

(R— S)2Zn  +  S* >•  (R— SS,)2Zn 

(R— SSx)2Zn >  (R— S)2Zn  +  S  (active) 

This  mechanism  affords  an  explanation  of  the  following 
facts: 

1 — The  metallic  salts  of  the  mercaptobenzothiazoles  are  faster 
curing  than  the  free  compounds. 

2 — Both  the  metallic  salts  and  the  free  compounds  are  faster 
curing  than  the  disulfides,  which  must  first  undergo  a  reduction 
to  the  mercaptans  before  functioning  as  accelerators. 

3 — All  mercaptobenzothiazoles  require  the  presence  of  zinc  or 
lead  oxides  for  the  development  of  full  accelerating  power. 

4 — The  metallic  salt  of  the  mercaptan  is  therefore  assumed  to 
be  the  active  agent.  Excess  metallic  oxide  must  be  present 
at  all  times  to  reform  the  salt  if  it  should  be  decomposed  by  the 
action  of  heat  or  hydrogen  sulfide.  Evidence  of  such  decomposi- 
tion is  to  be  found  in  the  low  curing  power  of  these  salts  in  the  ab- 
sence of  metallic  oxides,  and  their  superior  power  when  an  excess 
of  the  oxide  is  present. 

While  it  cannot  be  shown  that  sulfur  split  off  from  poly- 
sulfides  is  an  active  form,  it  has  been  proven  that  the  sulfur  in 
trithioozone  is  particularly  active,6  and  it  is  inferred  that  the 
sulfur  yielded  by  the  decomposition  of  the  polysulfides  is 
similarly  active. 

The  polysulfide  theory,  as  given  above,  appears  to  offer  the 
best  explanation  of  the  existing  facts.  Doubtless,  both  the 
mechanisms  given  above  function  in  specific  cases.  It  is  also 
probable  that  no  simple  explanation  of  activation  will  be  found 
which  is  equally  applicable  to  all  types  of  accelerators.  Fur- 
ther work  is  being  done  to  determine  the  validity  of  each  of 
the  reactions  involved  in  the  above  mechanisms. 

11  Romani,  Caoutchouc  gutta-percha,  19,  11626  (1922). 


27 


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28 
SUMMARY 

Mercaptobenzothiazole  and  its  substituted  derivatives  show  the  follow- 
ing relative  activity  as  accelerators  of  vulcanization:  (1)  3-methyl,  (2) 
3,5-dimethyl,  (3)  unsubstituted,  (4)  4-methyl,  (5)  5-methyl,  (6)  5-ethoxy, 
(7)  5-methoxy.  Several  compounds  with  a  structure  analogous  to 
1-mercaptobenzothiazole  were  prepared  and  their  accelerating  values  de- 
termined. The  mercaptobenzothiazoles  have  no  value  as  accelerators 
except  in  the  presence  of  zinc  oxide.  The  zinc  and  normal  lead  salts  of 
these  thiazoles  are  faster  curing  than  the  corresponding  free  thiazoles. 
The  zinc  salt  of  the  l-mercapto-5-methylbenzothiazole  and  the  normal 
lead  salt  of  l-mercapto-3-methylbenzothiazole  gave  the  highest  accelerat- 
ing values  of  any  of  the  compounds  tested.  The  disulfides  have  a  lower 
curing  power  than  the  corresponding  free  thiazoles.  The  accelerating 
power  of  the  mercaptobenzothiazoles  is  directly  connected  with  the  atomic 

grouping  — 5 — C — SH.  Any  alteration  in  this  grouping  removes  almost 
entirely  the  power  to  accelerate  rubber  vulcanization.  The  mercapto  group 
is  more  essential  to  the  accelerating  action  than  the  sulfur  atom  of  the 
thiazole  ring.  Both  are  necessary  to  develop  the  highest  accelerating 
power.  Aliphatic  mer  cap  thiazoles  show  marked  accelerating  power  but 
are  inferior  to  the  mercaptobenzothiazoles.  The  metallic  salts  of  the 
mercaptobenzothiazoles  are  assumed  to  be  the  active  agents  in  producing 
acceleration. 

The  existing  theories  for  the  mechanism  of  acceleration  by  mercapto 
compounds  have  been  given.  The  results  obtained  in  the  work  are  corre- 
lated with  the  polysulfide  theory. 

ACKNOWLEDGMENT 

The  writer  wishes  to  acknowledge  his  indebtedness  to  Dr.  C.  E-  Boord 
under  whose  direction  this  work  has  been  carried  out.  His  unfailing  cour- 
tesy and  optimistic  spirit  have  been  manifest  at  all  times;  and  his  helpful 
and  kindly  advice  has  helped  to  make  the  research  a  real  pleasure. 

The  writer  also  wishes  to  acknowledge  the  cooperation  of  the  Goodyear 
Tire  and  Rubber  Company,  as  expressed  through  Dr.  B.  B.  Spear,  for  the 
loan  of  apparatus  and  the  testing  of  samples.  Further  acknowledgment 
is  due  Mr.  C.  W.  Bedford  and  Dr.  W.  J.  Kelly  for  valued  suggestions,  and 
to  Mr.  C.  M.  Carson  for  assistance  rendered. 


AUTOBIOGRAPHY 

I,  Lorin  Beryl  Sebrell,  was  born  at  Alliance,  Ohio,  November  19,  1894. 
My  elementary  and  secondary  education  was  received  in  the  public  schools 
and  high  school  of  that  city,  and  my  undergraduate  college  training  at 
Mt.  Union  College.  I  received  the  Bachelor  of  Science  degree  from  the 
same  institution  in  1916,  and  in  1917,  I  was  granted  the  degree  of  Master 
of  Science  by  the  Ohio  State  University.  During  the  war  I  was  connected 
with  the  Research  Division  of  the  Chemical  Welfare  Service.  After  the 
war  I  served  one  year  as  instructor  in  chemistry  at  Case  School  of  Applied 
Science.  In  1919  I  entered  the  employ  of  the  Goodyear  Tire  and  Rubber 
Company  as  a  research  chemist,  holding  this  position  until  February, 
1921,  when  I  left  to  resume  graduate  work  at  the  University  of  Wisconsin. 
I  returned  to  the  Ohio  State  University  in  January,  1922,  and  completed 
the  requirements  for  the  degree  of  Doctor  of  Philosophy  at  the  end  of  the 
summer  quarter,  1922. 


54 5101 


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